Start with a dollar.

Fold the top corner down like this.

Unfold and fold down the other corner like so.

You should have something like this now.

You should have something like this. And in the interest of time, repeat all steps for the other side of the dollar. You may proceed to the next step once you have done so.

It should look like this.

You are now done. If you want, you can lay something heavy on it or stick it in a heavy book for a while until it becomes flat.

This is the final result. :D